How to measure time difference between 2 signal changes in verilog?

Question

There are two signals sig, enable - and I wanted to find the time difference after which enable toggles after sig falls. ( >Sig Low to Enable toggle< time)

I understand that always@() block can't be nested inside another, so I tried the following approach:

  realtime toggletime ;
  realtime sig_low;

  always@(negedge sig) begin
   sig_low = $time ;
           @(enable)begin
               toggletime= $time - sig_low ;
               end
  end

My idea was to trace sig, and whenever it goes low, note the time (in sig_low) and from that point, trace enable and whenever it changes note the difference between that point and the earlier sig_low value. But this isn't giving me the required result. Is there some other way of doing this?

*The sig signal is a periodic pulse that goes low say, every 500us. enable is a signal that toggles at sporadic intervals. I am trying to measure the time elapsed after the latest sig low after which enable toggles.

*Both are synchronous to the same clock. They don't trigger at the same time. If there are multiple neg edges, only the last edge is of interest.

Answer 1

The code you wrote should work. The only possible problem is that you should use $realtime instead of $time if the current timescale is greater than 1ms. That prevents time from being rounded to an integer.

I can rearrange the begin/end blocks to better explain how it executes.

   always begin
       @(negedge sig) // wait for sig to goto 0
       sig_low = $realtime ;
       @(enable)      // wait for enable to change its value
       toggletime= $realtime - sig_low ;
    end

This is functionally equivalent to what you wrote.